1. Field of the Invention
This invention relates to earth bits for drilling.
2. Description of the Related Art
An earth bit is commonly used for boring through a formation to form a borehole. Such boreholes may be formed for many different reasons, such as drilling for oil, minerals and geothermal steam. There are several different types of earth bits that are used forming a borehole. One type is a tri-cone rotary earth bit and, in a typical setup, it includes three earth bit cutting cones rotatably mounted to separate lugs. The lugs are joined together through welding to form a bit body. The earth bit cutting cones rotate in response to contacting the formation as the earth bit body is rotated in the borehole. Several examples of rotary earth bits are disclosed in U.S. Pat. Nos. 3,550,972, 3,847,235, 4,136,748, 4,427,307, 4,688,651, 4,741,471 and 6,513,607.
Some attempts have been made to form boreholes at a faster rate, as discussed in more detail in U.S. Pat. Nos. 3,250,337, 3,307,641, 3,807,512, 4,502,552, 5,730,230, 6,371,223 6,986,394 and 7,377,338 as well as in U.S. Patent Application No. 20050045380. Some of these references disclose using a percussion hammer to apply an overstrike force to the earth bit. However, it is desirable to increase the boring rate when using the percussion hammer, and to reduce the amount of damage to the earth bit in response to the overstrike force.